The use of RedirectAttributes was documented in four places in the
reference documentation. This commit merges some of the places and links
properly between them.
Issue: SPR-12759
Generally update chapter and add documentation for 4.2 including
the return value types ResponseBodyEmitter, SseEmitter, and
StreamingResponseBody.
Issue: SPR-12672
The upgrade to a more recent asciidoc format led to a bunch of of
broken Admonition as the text block now requires four equal signs while
previous versions tolerated only 3.
Issue: SPR-12791
Core and Web chapters are important chapters in the Spring Framework
reference documentation, and splitting them in multiple files will
help to evolve the documentation while not creating too many files.
Issue: SPR-12309
The reference documentation is now organized in asciidoc bookparts.
Bookparts sections can be also put in separate files, when their
size and theme are important enough to justify that.
Documentation Layout:
```
index
|-- overview
|
|-- whats-new
|
|-- core
| |-- core-beans
| |-- core-aop
|-- testing
|
|-- data-access
|
|-- web
| |-- web-mvc
| |-- web-view
|-- integration
|
|-- appendix
|-- appx-spring-tld
|-- appx-spring-form-tld
```
Supersedes and closes#641
Issue: SPR-12309
David Tombs
Sam Brannen
Rossen Stoyanchev
Brian Clozel
Stephane Nicoll